The invention utilizes the magnetostrictive properties of a metal alloy and the light modifying properties of a photoelastic optical fiber to provide a transducer for use in a fiber optic magnetometer. The principle of operation of the transducer is the transferral of strain from the magnetostrictive material to the optical fiber. The strain imposed upon the photoelastic optical fiber causes modifications in the phase of the light propagating along the optical fiber. The modification in the phase of the light propagating in the optical fiber may appear in two forms. In one of those forms, the phase modification is a phase shift caused by a change in the length of the optical path in the fiber. In the other form, the phase modification is a phase delay that appears as a difference in phase between orthogonal components of polarized light propagating in a stressed single mode optical fiber. In interferometry, phase shifts are usually detected with a two-fiber Mach-Zender interferometer whereas phase delays are usually detected with a single fiber interferometer often referred to as a polarimeter.
The crucial element in any fiber optical magnetometer is the transducer which converts magnetic field intensity into a strain and transfers that strain to the optical fiber. Because the sensitivity of the magnetometer is directly proportional to the length of the optical fiber to which the strain is applied, it is highly desirable to maximize the interaction length of the optical fiber.
In earlier transducers, the optical fiber was directly attached to a length of magnetostrictive ribbon. Because ribbon lengths were limited to 10 to 50 cm, the fiber-magnetostrictive ribbon interaction length of the earlier transducers was typically increased to about two meters by folding the fiber back and forth along the ribbon. The resonance structure (and therefore the frequency response) of such earlier transducers is ill defined and is dependent upon parameters, such as boundary conditions, that are difficult to control. Moreover, because the harmonic response of the ribbon can be large, the earlier transducers tended to introduce an unwanted signal at 2.omega. in the magnetometer's output.